1. Field of the Invention
This invention relates to optical information media such as read-only optical disks and optical recording disks.
2. Description of the Related Art
Optical information media such as read-only optical disks and optical recording disks have been required to have a higher capacity by increasing the recording density for the purpose of recording and storing an enormous amount of information as in the case of motion picture information. Extensive efforts have been dedicated to the research and development of the recording at a higher density to meet such request.
Under such situation, one proposal has been use of a smaller laser beam spot with a reduced diameter in the recording and reading as in the case of DVD (Digital Versatile Disk) by reducing the wavelength used in the recording/reading and increasing the numerical aperture (NA) of the objective lens of the recording/reading optical system. When the DVD is compared to CD, the DVD has realized a recording capacity (of 4.7 GB/side) which is 6 to 8 times larger than that of the CD by reducing the recording/reading wavelength from 780 nm to 650 nm and by increasing the NA from 0.45 to 0.6.
Use of a higher NA, however, invites decrease of tilt margin. Tilt margin is tolerance for the tilting of the optical information medium in relation to the optical system, and the tilt margin is determined by the NA. When the recording/reading wavelength is xcex, and the transparent substrate through which the medium is irradiated with the recording/reading beam has a thickness t, the tilt margin is proportional to
xcex/(txc2x7NA3) 
Tilting of the optical recording medium at an angle to the laser beam, namely, occurrence of the tilt results in the generation of wave front aberration (coma aberration). When the substrate has a refractive index of n and a tilt angle of xcex8, the wave front aberration coefficient is given by
(xc2xd)xc2x7txc2x7{n2xc2x7sin xcex8xc2x7cos xcex8}xc2x7NA3/(n2xe2x88x92sin2 xcex8)xe2x88x925/2 
These relations indicate that decrease in the thickness t of the substrate is effective when the tilt margin is to be increased with simultaneous suppression the generation of the coma aberration. As a matter of fact, tilt margin is ensured in the case of DVD by reducing the thickness of the substrate to about half (about 0.6 mm) of the thickness of the CD (about 1.2 mm)
A structure enabling further decrease in the substrate thickness has been proposed in order to realize high quality motion picture recording for a longer period. In this structure, a substrate having normal thickness is used as a supporting substrate for ensuring rigidity of the medium, and the pits and the recording layer are formed on its surface, and a light-transmitting layer in the form of a thin substrate having a thickness of about 100 xcexcm is formed on the recording layer. The medium is irradiated with the recording/reading beam through this light-transmitting layer. This structure enables drastic reduction in the thickness of the substrate, and high density recording by the use of a higher NA is thereby enabled. A medium having such structure is described, for example, in Japanese Patent Application Laid-Open Nos. (JP-A) 320859/1998 and 120613/1999.
The medium described in JP-A 320859/1998 is a magneto-optical recording medium, and this magneto-optical recording medium has a structure where in a metal reflective layer, a first dielectric layer, a magneto-optical recording layer, a second dielectric layer, and a light-transmitting layer are disposed on the substrate in this order. The medium described in JP-A 120613/1999 is a phase change optical recording medium wherein the medium is formed by disposing a reflective layer, a phase change recording layer, and a light-transmitting layer on the substrate in this order.
When a medium is provided with a light-transmitting layer of about 100 xcexcm thick, use of an objective lens having a relatively high numerical aperture (NA) of, for example, about 0.85 is enabled.
Increase in the NA, however, is associated with the decrease in the focal depth, and also, with the decrease in the distance between the light-transmitting layer and the objective lens (working distance). To be more specific, in a system with the NA of 0.85, the focal depth is xc2x10.3 xcexcm and the working distance is 100 to 300 xcexcm.
The working distance can be increased by increasing the diameter of the objective lens. The increase in the diameter of the objective lens, however, should be avoided in view of reducing the size of the optical pickup. As a consequence, increase in the NA invites increased risk of the contact of the optical pickup with the light-transmitting layer due to the reduced working distance, and hence, increased risk of the failure in the light-transmitting layer. In the meanwhile, the focal depth decreases in proportion to the square of the numerical aperture NA, and increase in the NA is likely to invite instability of the focus servo. As a consequence, scratches in the light-transmitting layer formed by the contact of the layer with the optical pickup is likely to result in an increased risk of focus servo error. By the way, the objective lens of the optical pickup is generally formed from a resin or glass, and it is commonplace to provide a protector around the objective lens to thereby avoid the objective lens from becoming in contact with the medium. In the case of an objective lens comprising a resin, for example, a protector is integrally formed with the objective lens for the purpose of protecting the lens surface. In the case of the glass, a resin protector is mounted on the lens. Due to such design, it is the protector that becomes in contact with the medium in the case of the contact of the optical pickup with the medium, and the objective lens is protected from scratches. Such contact of the protector with the medium, however, leaves scratches in the light-transmitting layer, and even if the lens should escape from failure, recording and reading may become impossible by the poor focus servo.
When a high data transfer rate is to be achieved in an optical disk system, the optical disk should be rotated at an extremely high speed. For example, in a system wherein the recording/reading wavelength is about 400 nm, the numerical aperture of the objective lens of the recording/reproducing system is about 0.85, and the recording capacity is 22 GB/side, the maximum rotation speed of the optical disk should be set at about 2500 rpm at the data transfer rate of 35 Mbps, and at about 5000 rpm at the data transfer rate of 70 Mbps although the precise speed may vary depending on the type of the recording and the formatting.
An optical disk which has been loaded on an optical disk drive inevitably has a considerable eccentricity, that is, a considerable mass eccentricity in view of its removability which constitute the major feature of the optical disk, and the eccentricity may be empirically as much as about 0.01 gm at the maximum. Rotation of such eccentric disk is inevitably associated with some vibration, and while the vibration is not a serious problem at a relatively low rotation speed, rotation at a high speed of, for example, over 3000 rpm invites drastic increase in the vibration due to the eccentricity, and the resulting large surface vibration leads to an increased focus servo error. The vibration of the optical disk due to the eccentricity can be suppressed by increasing the force of clamping of the optical disk by the drive. Increase in the clamping force, however, is associated with an increase in the stress the optical disk, and such increase in the stress often exaggerates the surface vibration. In addition, an optical disk which is a removable medium should be recordable/readable on every type of optical disk drive. In other words, an optical disk is required to exhibit good recording/reading properties without relying on the clamping force of the drive.
In view of such situation, an object of the present invention is to provide an optical information medium comprising a supporting substrate, an information recording area on the supporting substrate, and a light-transmitting layer covering at least the information recording area, wherein the information recording area is irradiated with the recording/reading laser beam through the light-transmitting layer, and wherein the light-transmitting layer is highly scratch resistant upon contact with the optical pickup. Another object of the present invention is to provide an optical information medium which has a scratch-resistant light transmitting layer, and which experience reduced vibration upon high-speed rotation without suffering from problems in focus servo and tracking servo.
Such objects are attained by the present invention which provides a novel optical information medium. The medium includes a supporting substrate, an information recording area on the supporting substrate, and a light-transmitting layer covering at least the information recording area. The information recording area is irradiated with the recording/reading laser beam through the light-transmitting layer, and the surface of the medium on the side of the laser beam incidence above the information recording area has a dynamic coefficient of friction of up to 0.4.
According to one aspect of the invention, at least a part of the surface of the medium that is on the side of the laser beam incidence and that comes in contact with a clamping mechanism of the optical information medium driven has a dynamic coefficient of friction greater than that of the part above the information recording area. According to another aspect of the invention, at least a part of the surface of the medium that is on the side incident the laser beam and that comes in contact with a clamping mechanism of the optical information medium drive, has a dynamic coefficient of friction greater than 0.4.
According to further aspects of the invention, the light-transmitting layer has a thickness of 30 to 300 xcexcm, and a lubricating layer containing a lubricant is present on the surface of the medium on the side incident the laser beam incidence, at least in the part above the information recording area. Additionally, the light-transmitting layer may contain a lubricant in every part thereof. The lubricant may be a fatty ester lubricant, a silicone lubricant, and/or a fluorolubricant.